1. Field of the invention
The invention relates to control systems and, more particularly, to stored-program controls comprising devices employed for controlling or regulating a machine or system.
2. Description of the Related Art
One of the principal requirements placed on stored-program controls is realtime (RT) capability, i.e., a system has to react to an event within a specified timeframe, i.e., within the time that processes take in the real world.
Systems of this kind that can run on modern PC systems now also exist. These, guarantee on the system side via a realtime operating system, thus an operating system having additional realtime functions for ensuring total adherence to time-related conditions and the process behavior's predictability, a realtime behavior of up to 500 μs, for instance.
Any interruptions to realtime behavior due, for instance, to higher-priority software and/or hardware interrupts (i.e., system-management interrupts) result in a “blackout” of the realtime operating system. The realtime operating system's time system will then be brought to a complete halt. The realtime operating system's time will resume after the interrupting event as though no interruption had occurred. As a result, it will not be possible for the operating system to adhere to reaction times. If this time is exceed, moreover, it will not be detected on the system side.
System-management interrupts (SMIs: Top-priority interrupts realized in the PC firmware) already pose a major problem for realtime operating systems. These kinds of interrupts are triggered on the firmware side in the case of, for example, certain temperatures in the processor (e.g., over temperature, threshold temperatures) or by PC function keys (such as screen switching and adjusting volume or brightness). SMIs can also be triggered via a USB interface. SMIs can last from a few milliseconds to, in some cases, more than 200 ms. Realtime operating systems are unable to accommodate interrupts of this type and thus lose their realtime capability. The user is unlikely to notice this until the control suffers “blackouts” and production outages occur.
Interruptions to the realtime operating system that result in one of the above-described blackouts, such as an SMI or a process (outside the realtime operating system) that uses the CPU or, as the case may be, system resources with higher priority, are at present not detected by the realtime operating system.